A lead frame forms the base or skeleton of an IC package, providing mechanical support to semiconductor dies during assembly into a finished package. A lead frame typically includes a die paddle for attaching a semiconductor die, and leads providing the means for external electrical connection to the die. The die can be connected to the leads by wires, e.g., through wire bonding or tape automated bonds. Lead frames are typically constructed from an electrically conductive material, such as copper or aluminum. The electrically conductive material may be provided in the form of a flat sheet metal. The features of the lead frames may be defined by forming openings in the flat sheet metal. The flat sheet metal can be patterned with a plurality of identically openings so as to form lead frame strips, i.e., interconnected strips used to package a number of semiconductor dies in a common process. Each lead frame strip includes a number of unit lead frames, with each unit lead frame having the die paddle and lead construction described above.
After completion of the assembly process, semiconductor dies attached to the die paddles are usually tested after separation of the unit lead frames from the lead frame strip, e.g., by punching. In other words, the semiconductor dies can be individually tested after singulation of the unit lead frames. Alternatively, the packaged semiconductor dies may be tested while still being physically supported by the lead frame strip using tie bars. This is commonly referred to as lead frame strip testing. In this technique, separation of the unit lead frames from the lead frame strip occurs after lead frame strip testing. However, the tie bars are formed from the same material as the die paddle, and are part of the unit lead frames. This is problematic for applications in which the die paddles serve an electrical connection function, e.g., in DSO (dual small outline) packages in which the exposed die paddles provide an electrical connection to the backside of semiconductor dies attached to the die paddles. In this case, the tie bars electrically short the die paddles to the lead frame strip and to other die paddles attached to the same lead frame strip, complicating the electrical testing process. Electrical isolation is also required for other lead frame processing such as partial plating and electrical charge processes.